This recipe adapted from Cook's Illustrated tastes divine!
Shred the carrot, celery and leek into small pieces.
Heat butter in large sauce-pan to medium-high, when foaming subsides, add vegetables and cook until well-browned, about 7 minutes. Reduce heat to medium. Stir in flour and cook until brown and fragrant, about 5 minutes. Whisk in both broths and bring to a boil, skimming off any foam. Reduce heat to low, add bay leaf, thyme, and peppercorns. Simmer, stirring occasionally, until thickened and reduced to 3 cups, usually 20 - 25 minutes.
Strain gravy through fine-mesh strainer into saucepan. Use in any other recipe that requires gravy.